In many industrial processes, the water concentration of flowing gas streams must be measured and analyzed with a high degree of speed and accuracy. Such measurement and analysis is required because the water concentration is often critical to the quality of the product produced. Consequently, many complex and sophisticated devices are available for measuring water in gases.
These devices typically incorporate an electrolytic cell operating under the principles of Faraday's Law, although other types of hygrometers based on other principles also exist. Basically, an electrolytic cell consists of a hollow glass tube with at least two electrically isolated electrodes (wires) helically wound in parallel around the inside and covered with a hygroscopic film. The electrodes cover approximately half of the surface area of the inside wall of the cell. The gas to be analyzed enters the cell at a known flow rate and the film absorbs all moisture molecules present in the gas flow. A voltage is supplied across the electrodes, which electrolyzes the moisture in the film. The current generated measures the rate at which the moisture molecules are electrolyzed. Once equilibrium is reached, the rate at which moisture molecules enter the cell will exactly match the rate at which such molecules are electrolyzed. Consequently, the water concentration in the gas will be known without any further calibration.
Such devices are generally unable, however, to measure extremely low concentrations of water with a response time sufficiently short to accommodate many applications.
For example, the electrolytic cell described in U.S. Pat. No. 4,800,000 to D. A. Zatko, incorporated herein by reference, is sensitive to water concentrations on the order of about 2,000 to 5 parts per billion by volume. That cell reaches, following a change in the entering moisture concentration, about ninety percent of the final equilibrium value in about five minutes--even for changes as small as 1.5 ppm. Although such levels of sensitivity and response time are impressive, improvement is desirable.
Specifically, the known electrolytic devices use a packing material such as an epoxy filler to mechanically fix the detection cell in place. The epoxy also serves as an electrical insulator for the electrodes and as a leak-tight barrier between the entrance and exit of the actual detector, which is typically the hollow glass tube of the cell. The epoxy barrier assures that the sample gas will flow only through, and not around, the glass tube of the cell. Finally, the epoxy provides a leak-tight barrier where the electrical connections penetrate through the metal housing of the cell body.
As a result of its many functions, the packing material is present in the vicinity of both the inlets and outlets of the detection cell. Such materials are known to be relatively porous and to absorb or emit water from or into the gas stream. It is known that the outgassing, absorption, and desorption properties of packing materials, such as plastics, epoxy, and the like, form an obstacle to reaching low water concentrations in high purity gas systems.
Several possible mechanisms limit the performance of these hygrometers through the presence of such contaminating materials. One such mechanism, outgassing, occurs when residual water contained in the packing material migrates out of the material and eventually joins the flowing gas stream under test. This contamination prevents the instrument from reaching lower detection limits.
A second mechanism limiting the response time is absorption of the moisture present in the gas stream by the packing material and desorption (or emission) from that material into the gas stream. On the one hand, this mechanism allows previously absorbed moisture to desorb back into the gas flow upon a change from wet to dry gas. When the gas under analysis is relatively dry and has been analyzed for a relatively long time, on the other hand, the surface of the packing material may become dry. Consequently, the material may absorb moisture from the gas at the inlet for some time before the moisture present in the gas can reach the actual detector (and enter the glass tube in an electrolytic cell). In both cases, the moisture measurements of the cell will take a long time to reach equilibrium under the changed conditions.
For an electrolytic cell, the negative effects caused by the presence of epoxy at the inlet side of the cell are seen directly. But the presence of epoxy at the outlet side also has adverse effects. Contamination at the outlet side occurs through back diffusion of moisture against the small sample gas flow exiting the glass tube.
In general, the configuration of the prior art devices contributes to the unfavorable mechanisms of outgassing, absorption, and desorption. These mechanisms, in the case of hygrometers, increase the response time and limit detection of very low concentrations. Further, these mechanisms are temperature and pressure dependent.